Thank you, but I think I'm really asking how exactly that "disabling" should be done. The UI has changed repeatedly over the years, and it's not clear to me tha

Thank you, but I think I'm really asking how exactly that "disabling" should be done. The UI has changed repeatedly over the years, and it's not clear to me that things like stopping the front-end app from loading, force-closing it if it's already open, or changing settings related to mail accounts would necessarily be sufficient here. Presumably deleting all of the mail accounts on any affected phone would do it, but "You just need to delete everything and then you can set it all up again later" isn't great advice to have to give to a non-techie relative if it's not necessary to go that far.